Differential transformer



Jan. 9, 1951 w. D. MACGEORGE DIFFERENTIAL TRANSFORMER Filed April 24. 1947 To FL ow CONTROL 0R INDICATOR lNl/EN TOR u ILL/AM 0. Magus-ms:

ATTORNE Y5 l'llll' Patented Jan. 9, 1 951 DIFFERENTIAL SFORMER William D. Macgeorge, Havertown, Pa., assignor to Automatic Temperature Control 00., Inc., Philadelphia, Pa., a corporation of Pennsylvania Application April 24, 1947, Serial No. 743,540

8 Claims. (Cl. 171-777) This invention relates to differential transformers. I

There are many situations in which a differential transformer is useful for effecting controls of various electrical circuits, especially where it is desired to obviate make and break contacts, and in a co-pending application, Serial Number 740,507, filed April 9 1947, I have illustrated such a differential transformer in connection with relay circuits, for effecting predetermined actuation of relays, when a movable armature or core attains certain attitudes relative to the transformer. In the illustrative transformer system of said co-pending application, the secondaries thereof were oppositely wound, series-connected and axially aligned symmetrically on opposite sides of a primary or exciting coil also aligned with the secondaries, so that a single armature or core could pass axially through and relative to all of the aligned coils to vary the output of the secondaries between voltage of A phase, a null output and voltage of B phase, ac-

cording to the relative positioning of the movable armature or core and the coils. A transformer of this type is extremely critical in operation in the sense that the range of relative movement of the armature or core to pass from one unbalanced condition of the secondaries to the other unbalanced condition of the secondaries is very short indeed. The transformer device is therefore primarily available for association with a movable element having as the actuating device of the armature relatively short paths of total travel.

There are many situations in which for efficiency a different form of differential transformer should be used, especially in connection with circuits such as are illustrated in said application. lllustratively, a modified differential transformer should be used where the movable element actuating the core has a long travel stroke.

It .is among the objects of this invention: to improve the art of differential transformers; to provide a differential transformer with a permanent iron core for one of two series-connected oppositely wound secondaries; to provide a differential transformer in which a movable core is provided having along stroke in which the length of travel of the core to and from the null point may be much greater than has previously been considered possible; to provide a differential transformer with two cores, one of which is relatively permanent but adjustable in setting, and the other of which is bodily movable to effect changes in the output from the transformer; to

provide a differential transformer with two cores,

one of which may be considerably longer than the other, the length and stroke of the latter being limited only by the dimensions of its actuating device; and to provide other improvements and objects as will appear as the description proceeds.

In the accompanying drawings forming part of this description:

Fig. 1 represents a fragmentary diagrammatic section of an illustrative type of element movable in accordance with the condition of a conditioned device such as a rotameter, and showing schematically the wiring of the transformer of this invention.

Fig. 2 represents a transverse vertical section of the transformer itself in accordance with the preferred embodiment thereof.

Fig. 3 represents a horizontal section on line 3-3 of Fig. 2;

Fig. 4 represents a side elevation thereof.

The transformer I0, according to this invention is preferably formed of upper and lower plates of insulating material, such as the phenolic resin plates I3 and I4, engaging the ends of parallel spaced tubular spools I5 and [6, the axial apertures of which are in registration with corresponding apertures l1 and IS in the plates 13 and I4. Secondary coils 20 and 2| are wound on the respective spools and are oppositely connected in series so as to have substantially a zero output from the terminals of these secondaries, when the magnetic reluctance of the flux paths through these secondaries is equal, for connection with and into a suitable circuit. A relay circuit such as is described in said application is preferably used. Illustratively, the output of the secondaries is led to the primary coil 22 of a transformer 23, having a secondary which is preferably center-tapped for control purposes in any desired relay or other circuit. A primary winding 25 is provided which is wrapped about the secondaries on the spools between the plates,

and has terminals for connection to a suitable source of A. C., such as line L1, L2. In one of the spools, iron or related magnetizable material is more or less permanently mounted, and this, illustratively, may comprise a screw 26, as a permanent core disposed in the coil 2|. The screw 26 may be replaced as desired with bigger or smaller screws or screws of different lengths, as it may be desired to predetermine the characteristics of the differential transformer. An elongated movable core which may comprise a relatively thin iron rod 21 or other magnetizable element is provided, having the free tip end 29, which core is preferably of substantially uniform mass distribution per unit of length, and arranged for guided movement through registering apertures ll and the hollow spool l6 axially of the coil 2%. The movable core is predeterminedly of such mass distribution relative to that of the fixed core 26, that with the core 21 suitably advanced into and more or less through the coil 20, there is an operative length thereof extending from the free end 29 upwardly axially or longitudinally of the armature 21, which is of maximum predetermined preponderance over the mass of the fixed armature 26 by which the induced voltage, say, of B phase, in the coil 2% is of a maximum preponderance over the opposing voltage of A phase induced in coil 2 by thefixed core, so that the resultant output is of a maximum amplitude of B phase voltage. Because of the substantially uniform mass distribution of the movable core 2'! and the preferred elongation thereof, any further inward or downward movement of the movable core, moving the tip end 28 below or away from the coil 20, continues to expose a similar proportionate operative portion of the moving core to the secondary 20 so as to maintain the said maximum resultant output of B phase voltage. In other words, the core 21 is preferably longer than the effective length ofthe field of the secondary 2e, so that-after the attainment of the maximum preponderance of operative mass of the moving core 21, the same electrical value of the movable core is maintained, despite the physical relative shift of the core and secondary coil 20 in the proper direction as new operative lengths of similar mass continue to be exposed to the effective length of such field. This important characteristic enables the use of a core of practica ly infinite total length, any given similar length thereof constitutngan operative portion which has the same electrical effect on its secondary as the operative portion terminating at the very tip end thereof, the only limits on the length of core 2'! being practical, as determined by the maximum stroke of the element which moves as a function of the change of condition of the instant or particular conditioned device.

This gives. rise broadly to three output conditionsof the, transformer, the first of which exi ists with the moving core 2'! extending through and beyond its coil it, to any degree sufficient to expose the necessary proportionate portion of its length to the field of coil 2% so as to induce a voltage of B phase preponderatingin the output of the coupled secondaries. This maximum. out put maintains as the movable core, is, i1lustratively, progressively moved upwardly in the coil 2Q, until the tip end 29 thereof coincides with the end of the effective field of the secondary. Thereafter, and with further progressive movement in the same direction, the output voltage of B phase begins to decrease, until that point is reached at which the mass of the moving core in coil 20 with a shorter extent of the core 21 in the effective field of coil 2t, balances the mass of the fixed armature 2% in coil 2|, and the output be comes, null, because the voltages of coils 20 and 2| are mutually cancelling at this point. This is the second output conditionof the transformer. With further outward or progressive movement relatively of the movable core and the transformer, as the withdrawing core 21 exposes a smaller mass to the effective field of its coil 20 than the fixed core 26 does to its secondary 2|,

4 there is the beginning of an output from the coupled secondaries of A phase voltage, which progressively increases and reaches a maximum amplitude when the movable core has been withdrawn to a point such as to cause the tip 29 to leave the effective field of coil 20 and therefore the core has no magnetic effect upon. the secondary 20. Of course, with the movable core completely withdrawn, and then progressively i advanced toward, into, and through its secondary 20, there will be a reversed sequence of output conditions beginning with the maximum output of A phase, through the null point, to the maximum output of B phase, which latter maintains 5 during further continued progression of the movable core in the same direction. Usually in setting the assembly it is preferred that the trans former be vertically adjusted along the path of travel of the movable core, until the tip end 29 of the core is so located in its coil 2o as to estab lish the null point at a desired predetermined position of the parts, from which movement of the movable core gives an output varying in phase according to the direction of its movement and varying in amplitude according to the degree of its movement. It is, of course, understood that the transition from one condition to another is not abrupt, but is gradual and substantially linear.

As an illustration of a mount for the movable core, reference may be made to the schematic illustrativedisclosure of Fig. 1, in which a rotameter Si! is: provided, arranged for movement vertically between convergent walls 3| of a fiowmeter, in which the'rate of flow is the condition in which the rotameter responds. Withthe core 2-? coupled to the rotameter 3B the free end of the core 21 may have a maximum stroke vertically of the order of from one to twelve inches or more. To control the flow so as to maintain it at or about a predetermined constant, the output of the differential transformer is used as indicated by the control system of said co-pending application. It is only necessary in setting 5 the instrument to predetermine the null point of the transformer output. This is accomplished by preliminarily adjusting the transformer I0 vertically along the path of the movement of the free end of the armature 21, so. as to cause the null point in the output of the differential transformer "to occur where desired inthe stroke path of the core; carried by the rotarneter float. A mounting for such bodily shift of the transformer is shown in said application.

It being recognized that in some cases, as in the illustrative rotameterinstallation described, the upper end of the core 21' will be mounted in the path of fluid flow, a tube 32 of substantially non-magnetic material, such as of stainless steel, will be sealed into the lower end of the flowmeter by direct mechanical attachment or by suitable packing glands and the like, and have its upper end open to receive the core 21 as-well as the fluid, while the lower'end will be sealed so as to contain such fluid. The openings'in, the spool i6 and in the plates l3 and M will be so arranged as to permit the tube 32 to pass downwardly through the transformer and to permit relative vertical adjustments of the transformer I!) and the fixed tube 32, in setting the instrument.

The simplicity and cheapness of the transformer will beclear and the ease of. balancing by the. replaceability or adjustability of the permanent core 26, will be evident.

Havingrthus: described: my invention, I claim:

1.. Aidifierential transformer for operative:- as:- sociationzwith a; device; having-a; working; stroke of'ther order of an inch or'more; .comprising:: a primary exciting coil, two secondarycoils: wound and connected in series and: operatively associated with the primary so that the respective outputsof the Secondary coils are 180. outof phase. so that with equal voltages induced in the secondary coils there is a' nu-lloutput therefrom, a fixed core. of knowninductive-efiectassociated with one: of said coils forproducing' a. predetermined voltage output from-said coil, and a core elongated relative to said fixed core havinga free end: and mounted Ior movement relative to theother of said. coilsandbeing of substantially uniform efiective' massdistribution longitudi nally of the core and arranged when properly located relative to sai'd'other'coi'l to expose-such mass: thereof to. said' other coil as to establish zero voltage output from the coupled secondary coils, said elongated core having such uniform mass' that: travel in: onedirection relative to --its coil permits production of a predominate volt-- age output from: said fixed" core coil; and travel in ther-opposite-direction produces a predominate voltage output in the elongated core coil, said last voltage becominga maximum when saidelongated core reaches a predetermined'position" different from said proper'location, said maximum voltage remaining constant throughout the travel of said elongated core'beyond said predeterminedposition'in the said last mentioned-direction;

2; A'- dififerential' transformer forv controlling an output, comprising two" series-connected'seoondary coils wound and connected so-tha-ttheir respective outputs are- 180 out'of' phase, each being hol ow and having an axis; a core of' known inductive massextended at least partially into' one ofsaid secondaries, an elongated core of appreciably greater inductivemass thanthat of said first mentioned core movable on a path to passthrough the other of said secondaries, a

primary exciting coil operatively associated with both coils for symmetrically energizing both secondariespsaid last: mentioned movable core being arranged for movement relative to its coil, starting with a smaller effective inductive mass equal to that of the first mentioned core through exposure of an effective inductive mass greater than that of" said first mentioned core: suehsns with a; continued axial movement in one-direction. to. create successively an output from: the coupled secondaries as a voltage and phase induced in that secondary coil. inwhichzthefirst mentioned core is disposed, a null output, and an output of voltage of the phase induced in the secondary coil relative to which the movable core moves.

3. A flowmeter comprising a device having a substantially linear stroke as a function of changes inrate of flow through the fiowmeter in the range of the fiowmeter, a differential transformer comprising a first and a second secondary coil the respective outputs of which are in mutually bucking opposition and said first secondary having an axial length appreciably shorter than said stroke, primary means for exciting said secondaries, an elongated core coupled to said device and movable axially along said stroke, means for guiding the core through and relative to said first secondary coil, said core having substantially uniform inductive mass distribution throughout its length and said device having a: holding: point at which thefree. end of theecore. has an established relation to said firstsseeondary at; which with a: given portion of the. movableicore of predetermined mass adjacent: to'. said end in inductive. relation to the. first secondary a predetermined voltage of given phase induced in'csaid. first secondary, a second core fixed relativeato the second secondary in inductive. relationtheretoand of inductive mass predeterminedly equal.v to the; inductive mass of said ,givenportioniof' said movable core inducing a predetermined voltage, of phase opposite to saidi given phase, both of: said predetermined voltages, being of the same amplitude so that the resultant output from. the, secondaries is null and constitutes: a signal of attainment of the holding point; said elongated core being movable in either direction with the device to move the said free end thereof from the established point to vary the inductive masses thereof relativeto that of said given portion in inductive relation to said first secondary and to vary the amplitude of induced voltage of given phase thereof relative. to the predetermined voltage output of the second secondary to vary the resultant output of the: secondaries in a signal of one sense; or the otherv according to the direction of deviation. of said device: from the holding point according to the phase and amplitude of the resultant output.

4. In flow controlled devices, a differential. transformer for operative association therewith comprising. a primary excitingcoil, two secondary 0011s, connected: so that their respective outputs arein bucking opposition, afixed core entering one secondary at one. end and. terminating-internally inspaced relation to. the other end of said secondary to produce a given inductive effect in said? secondary, an axially movable core for operative association with a. flow-controlled device passing through said other secondary for producing a varying inductive effect: therein ac cording: to its instant relation thereto as it is moved" by such device; said transformer having an output depending in magnitude and phase on the relative inductive effects: of" the respective cores; on. their respective secondaries.

5 In translating devices, aidifferential transformercompri'sing av primary exciting coil, two secondaries connected so that their respective outputs areinbucking-opposition; a relatively adjustable core: mounted in operative inductive relationxto; onesecondary and of adjustable inductive effect therein, a movable core having a tip end and. mounted for movement relative to the other secondary and being of varying inductive effect relative thereto according to the instantaneous relative position of the tip end thereof, said movable core being appreciably longer than said fixed core and of such inductive mass distribution compared to said adjustable core as to cause an electrical indication of the position of said tip end of said movable core.

6. In a flow meter the combination of a differential transformer comprising a first and a second side-by-side axially parallel secondary coils coupled in bucking relation to furnish an output, a primary coil embracing said secondaries to energize same, connections from the primary coil for a source of oscillatory voltage, a reservoir having an inlet and an outlet passage for the flow of liquid therethrough, a tube closed at the lower end and depending from the reservoir in communication therewith and extending axially through the first secondary coil, a member in an elongated armature core carried by said mem her having a free end and riding in said tube, said elongated core having an axial length'at least as long as said working stroke and having said free end positionable at a given point within and axially between the ends of said first secl ondary when said member is at the level indicative of the predetermined flow through the reservoir, and movable in both directions from said given point axiall out of said first secondary coil with appropriate changes in the level of said member, said first secondary having an output of given phase when said end is at said given point, and a fixed armature core in the other of said secondary coils for inducing an output of opposite phase and of amplitude such as to furnish with the output from said first secondary a resultant substantially null output from the secondaries when said end of said elongated armature core is at said given point within the first secondary coil, whereby an output preponderantly of one phase or the opposite phase caused by the movement of said free end from said given point produces an electrical signal.

7. In a control device the combination of a differential transformer comprising a first and a second side-by-side axially parallel secondary coils coupled in bucking relation to furnish an output, a primary coil inductively associatedwith said secondary coils to energize same, connections from said primary coil for a source of oscillatory voltage, an elongated armature core slidable in said first secondary and having a free end, a sensing member having a path of reciprocatory movement mounting said elongated core and having a stroke of movement longer than the axial extent of said first secondary coil so as to move said free end out of its coil in both directions with movements of the sensing member on its path, said first secondary having an output of given phase when the free end of said core is at a given point within said first coil coordinated with a desired setting of the sensing member, a fixed armature core in and shorter than the axial length of the second secondary coil for inducing an output in said second secondary of opposite phase and of amplitude to cancel the output from said first secondary and form a resultant substantial null output from the coupled secondaries when said free end of the elongated armature core is at said given point within the first secondary coil, whereby an output preponderantly 8,. of one phase or the opposite phase caused by the movement of the free end of said core from said given point produces an electrical signa1 as a function of deviations of the sensing member from its desired position.

8. A signalling system for signalling the location of the free end of a movable elongated inductive mass having a free end in relation to a datum setting for said free end, comprising a hollow secondary coil A, a hollow secondary coil B, a primary winding for exciting both coils, and

means coupling the coils in opposition so that their respective outputs are of substantially opposite phases and are bucking in forming a resultant signalling output, an inductive mass located in coil A and establishing a coil A output of substantially fixed known amplitude when the primary is energized with a known A. C. voltage, in combination with a movable elongated induc-" tive mass having a free end, means for guiding the elongated mass so that the free end thereof has a path of travel coaxial with and through coil B of predeterminedly greater length than the axial length of 0011 B, means for moving said elongated inductive mass relative to coil B to establish three ranges of amplitudes of coil B outputs, one said range being with the free end and part of said elongated inductive mass within coil B at which the amplitude of coil B output is substantially equal to the said known amplitude of coil A output to establish a substantially null resultant output as a signal of datum location of said free end relative to coil B, the, other ranges comprising movement of the elongated inductive mass so that the free end thereof moves in one sense or the other from datum whereby the amplitude of coil B output dominates or is subservient to the amplitude of coil A output to furnish a resultant output signalling the sense of departure of said free end from datum regardless of the degree of departure from datum inthe given sense b movement of said free end along its said path.

WILLIAM D. MACGEORGE.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,671,106 Fisher "May 29, 1928 1,954,996 Hehn Apr. 17, 1934 2,408,524 Mestas Oct. 1, 1946 2,427,866 Macgeorge Sept. 23, 1947 2,430,757 Conrad et a1. Nov. 11, 1947 

